1. Field of the Invention
The present invention relates to a lighting unit and a display device.
2. Description of the Related Art
For example, a display device such as a liquid crystal display device includes a lighting unit referred to as a front light or a backlight. The lighting unit includes a light source and a light guide plate. Light emitted by the light source enters the light guide plate. The light is propagated through the light guide plate while being repeatedly totally reflected within the light guide plate. The light leaves the light guide plate toward a display panel by means of a light discharging mechanism such as a set of prisms.
A cold cathode tube (fluorescent lamp) is used as the light source. The cold cathode tube is located at the side of the light guide plate together with a generally U-shaped reflector. Light emitted by the cold cathode tube and reflected by the reflector falls on the light guide plate.
Moreover, an LED (or LEDs) can be used as the light source. The LED is manufactured as an LED package having a semiconductor chip sealed with a resin. A mirror is formed behind a light emitting part of the LED package, and light is directed forwards from the LED package. However, the directivity of light of the LED is low, and light radially travels in forward and oblique directions at various angles. This poses a problem in that the usage efficiency of light emitted by the LED and entering the light guide plate is low.
Moreover, a certain type of lighting unit includes two light guide plates. A light source is located at the side of a first light guide plate, and the first light guide plate is located at the side of the second light guide plate. Light enters the first light guide plate, and then enters the second light guide plate after coming out of the first light guide plate. The light is propagated through the second light guide plate while being repeatedly totally reflected within the second light guide plate. The light leaves the second light guide plate toward the display panel by means of a light discharging mechanism such as a set of prisms. The thickness of the first light guide plate is the same as the thickness of the second light guide plate. The second light guide plate is bonded to a polarizer with a low refractive index layer, the refractive index of which is lower than that of the second light guide plate. In this kind of lighting unit, light exhibiting a large angular distribution leaks out of the second light guide plate and degrades the contrast offered by the display device. Consequently, it has been required that light exhibiting a high degree of parallelism enters the second light guide plate. Moreover, there is a problem that a part of light guide plate located near the light source causes degradation of the contrast.
Also, in the case where the light guide plate, the polarizer, and the liquid crystal panel are bonded to one another as a unit, and a low refractive index layer whose refractive index is higher than the refractive index of air but lower than the refractive index of the light guide plate is interposed between the light guide plate and the polarizer, light entering the light guide plate at an angle greater than a total reflection angle of the light guide plate is not propagated through the light guide plate but enters the liquid crystal panel, at a large angle, as it is. This light degrades the contrast offered by the liquid crystal panel, makes the contrast and the brightness non-uniform, and degrades the quality of the display.
When the light guide plate and the liquid crystal panel are bonded to each other with the low refractive index layer, of which the refractive index is lower than that of the light guide plate, light propagated in the light guide plate is not completely reflected by the interface but passes through the polarizing layer and enters the liquid crystal panel. The propagated light therefore causes degradation of the contrast.
Moreover, when the light guide plate, the polarizer, and the liquid crystal panel are bonded to one another as a unit, it means that hard plates are bonded to one another. This poses a problem in that the hard plates are liable to peel off. In particular, if dust is caught in the adhesive, the hard plates are liable to peel off.